Deep brain stimulation is a therapeutic technique comprising implanting a medical appliance known as a brain stimulator that serves to send electrical pulses to specific regions of the brain. For example, stimulation of the nuclei of the thalamus or of the hypothalamus can be used for treating motor disorders such as tremor, caused in particular by Parkinson's syndrome; in this context, reference can be made to the following articles:                P. Krack, P. Pollak, P. Limousin, A. Benzzaous, A. L. Benabid, The Lancet, Vol. 350, Dec. 6, 1977; and        A. L. Benadid et al., Acta Neurochirurgica suppl., Vol. 58, pp. 39-44, 1993.        
It has also been envisaged to stimulate the posterior hypothalamic nucleus for treating cluster headaches, periaqueductal gray matter for attenuating pain, and the ventromedial hypothalamus for treating certain kinds of obesity; in this respect, reference can be made to the following articles:                A. Takaki, S. Aou, Y. Oomura, E. Okada, T. Hori, “Feeding suppression elicited by electrical and chemical stimulation of monkey hypothalamus”, Am. J. Physiol. 262 (Regulatory Integrative Comp. Physiol. 31) R586-R594, 1992.        K. Sanyo, Y. Mayanagi, H. Sekino, M. Ogashiwa, et al., “Results of stimulation and destruction of the posterior hypothalamus in man”, J. Neureosurg 33, Dec. 1970, pop. 689-707.        C. Bielajew, J. Stenger, D. Schindler, “Factors that contribute to reduce weight gain following chronic ventromedial stimulation”, Behav. Brain Res. 62 (1994), pp. 143-156.        F. Brown, R. Fessler, J. Rachlin, S. Mullan, “Changes in food intake with electrical stimulation of the ventromedial hypothalamus in dogs”, J. Neurosurg. 60, pp. 1253-1257 (1984).        
Recently, a study has shown that electrical stimulation of the subgenual cingulate cortex, and more precisely of Brodmann area 25 (CG25) can be used for treating particularly severe and treatment-resistant forms of clinical depression [H. S. Mayberg et al., “Deep brain stimulation for treatment-resistant depression”, Neuron, Vol. 45, pp. 651-660, Mar. 3, 2005]. The stimulation method recommended is stimulation of the CG25 subgenual cortex by direct application, between the hemispheres, using electrodes presenting surfaces similar to those used for stimulating premotor cortexes in indications for refractory pain.
In any event, deep brain stimulation involves inserting a flexible lead into the patient's skull under guidance from a cannula and/or a rigid stylet, until the tip of said lead reaches the region of the brain that is to be stimulated. Close to its tip, the lead has electrodes (generally four electrodes) that are connected via a subcutaneous cable to a pulse generator appliance that is implanted under the patient's skin like a conventional cardiac pacemaker. The cannula and/or the stylet are extracted from the patient's skull after being used for inserting the lead, and the lead is then left in place for a duration that may be as long as several years.
A more detailed description of the procedure for implanting a lead for deep brain stimulation is given by the following document: “Medtronic—DBS™ lead kit for deep brain stimulation 3387 3389—Implant manual” from the supplier Medtronic Inc., downloadable from the following Internet site: http://www.medtronic.com/physician/activa/downloadablefil es/197928_b—006.pdf
Leads for deep electrical neurostimulation of conventional type are described for example in the above-mentioned document from the supplier Medtronic, and also in document U.S. Pat. No. 6,512,958.
The rectilinear shape of the lead is required by the need to make insertion thereof as little traumatic as possible for the patient. However, that has the drawback of allowing only a very small region of brain tissue to be stimulated, whereas in order to obtain more effective treatment it would be desirable to be able to act on a target of larger volume. Implanting a plurality of leads directed at distinct points of a common target region of relatively large volume is indeed possible, but that multiplies the risks and the side effects of the surgery.